1. Field of the Invention
The present invention relates generally to selective laser sintering and, in particular but not exclusively, to a metal powder composition for use in selective laser sintering wherein a three-dimensional object of a desired shape is made by irradiating an optical beam on a metal powder layer to form a sintered layer and by laminating such sintered layer one above another. The present invention also relates to a method of making the metal powder composition and to a three-dimensional object shaped with the use of the metal powder composition.
2. Description of the Related Art
A method of making a three-dimensional object with the use of selective laser sintering is known, wherein an optical beam (directional energy beam, for example, a laser) is first irradiated on a predetermined portion of a layer of metal powder composition to form a sintered layer. The sintered layer thus obtained is then covered with a new layer of metal powder composition, and the optical beam is irradiated on a predetermined portion of the new layer to form a new sintered layer, which is united with the underlying layer. These processes are repeatedly carried out to form a sintered article or three-dimensional object in which a plurality of sintered layers are firmly laminated one above another. This method makes it possible to obtain a shaped object of various states including a state in which there are so many spaces (holes) inside the shaped object and a state in which the metal powder composition has been melted substantially completely and then solidified, i.e., a state having a density (sintered density) of approximately 100% by controlling the energy density of the optical beam. For this reason, this method can be used to make a forming die that is required to have a smooth surface. In addition, this method enables a surface region of a shaped object to have a high density, an internal region to have a low density, and a region therebetween to have an intermediate density. In the case where the shaped object has a varying density, the shaping speed is not sacrificed for the smooth surface.
However, making such a shaped object having different surface and internal densities requires a metal powder composition having characteristics that differ from those of a metal powder composition to be used in normal powder sintering.
By way of example, the particle diameter of the metal powder composition has to be smaller than the thickness of each powder layer. A smaller particle diameter increases the packing density of the powder composition and the absorptivity of the optical beam during shaping and, hence, not only can the shaping density be increased but the surface roughness can also be reduced. However, a too small particle diameter sometimes causes cohesion of the powder composition, resulting in a reduction in packing density of the powder composition and making it impossible to uniformly form a thin powder layer.
Furthermore, in order for the shaped object to have a required strength, a portion irradiated with the optical beam and an underlying sintered layer have a large bonding area and a high adhesion strength. Also, the portion irradiated with the optical beam must not have a big rise or protrusion on the upper surface thereof. If such a rise is higher than the thickness of a powder layer to be formed thereon, formation of the powder layer sometimes becomes difficult.
In addition, because unnecessary metal powder adheres to the surface of the shaped object, good working properties are desired in machining the shaped object to remove the unnecessary metal powder to expose a high-density surface region.
As a matter of course, no large cracks must be present in the surface of the shaped object, and considering that a fluid medium such as cooling water is caused to flow through, for example, an injection molding die, no microcracks are desired in an internal structure thereof.
The metal powder composition irradiated with the optical beam partly or entirely melts and then solidifies by subsequent rapid cooling to turn into a sintered material. A high wettability during melting increases the bonding area between the molten material and adjacent sintered material, and a high flowability reduces the rise or protrusion. For this reason, the high wettability and the high flowability are desired.
In the light of the above, the inventors of this application proposed a metal powder composition as disclosed in Japanese Laid-Open Patent Publication No. 2001-152204. This metal powder composition contains chrome molybdenum steel powder, phosphor copper or manganese copper powder, and nickel powder. Chrome molybdenum steel is employed for its strength or toughness, phosphor copper or manganese copper is employed for its wettability or flowability, and nickel is employed for its working properties.
The metal powder composition referred to above offered generally good results in obtaining a shaped object having a density difference between a surface region and an internal region, but still has room for improvement in wettability, flowability and working properties (machinability).
As shown in FIG. 11 depicting a photograph, at a magnification of 25, of a section of a shaped object obtained from a conventional metal powder composition, a portion sintered at a high density has microcracks formed therein, which in turn deteriorate the shaped object particularly when it is used as a forming die.